1. Field of the Invention
The present invention relates to microwave systems, typically communication systems, such as find use in the microwave communications bands, e.g. the 21.2 to 23.6 GHz band. The invention further relates to a novel single balanced mixer for effecting a stable, low cost, conversion to an intermediate frequency suitably low (240 MHz) for convenient amplification. In a mixer operating at these frequencies, efficient transmission paths and efficient filtering must be provided. The disclosed transmission paths include waveguides and a dielectric substrate with metallizations thereon installed within the waveguide in what may be called a "Finline" construction.
2. Description of the Prior Art
In microwave communications to which the invention has application, the conventional system incorporates two transceivers communicating over a microwave link, each transceiver incorporating both a transmitter and a receiver connected to an antenna functioning in both transmission and reception. The system may also take the "Simplex" form in which only a single transmitter and a single receiver are involved.
The receiver in such systems incorporates a mixer as the first active element usually preceded by a low loss band pass filter to remove image frequencies and other interference. The noise figure, a critical parameter of such receivers, is determined primarily by the mixer.
A known mixer having this application is disclosed in the U.S. Pat. No 4,418,429 granted Nov. 29, 1983 to Clayton R. Roberts, and Assigned to the present Assignee. The mixer disclosed in the patent was a single balanced mixer coupled to the signal source and local oscillator with waveguide transmission paths and using microstrip internal transmission paths. In practical embodiments of the patented mixer, typical double sideband noise figures of seven to eight decibels were obtained.